156 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 245. — Superficial and second layers of the abdominal and pectoral muscles seen from in front. 

 On the right side the pectoralis major and the obliquus abdominis externus have been removed. 



Between the rectus capitis posterior major and the obliquus capitis superior and inferior is situated a small triangle 

 (the suboccipital triangle) in which is exposed the posterior arch of the atlas, crossed by the vertebral artery. 



THE FASCIJE OF THE BACK. 



In the dorsal region there are but two fasciae worthy of note, the lumbodorsal fascia and the 

 nuchal fascia. The upper layer of the flat muscles of the back is covered only by the general 

 superficial fascia. 



The lumbodorsal fascia (Figs. 236 to 238) is composed of two layers, the strong posterior 

 layer, which is superficially situated in the back and forms the aponeurosis of the latissimus and 

 the serratus posterior inferior, and the anterior or deep layer, which is developed only in the lumbar 

 region, where the long muscles of the back are included between the two layers. 



The posterior layer (Figs. 236 and 238) covers the sacrospinalis from behind and extends 

 above the uppermost portion of the latissimus to the inferior margin of the aponeurosis of the 

 serratus posterior superior. Although the fascia becomes considerably thinner after it ceases to 

 be the aponeurosis of the latissimus, it usually still contains distinct tendinous slips. In the 

 thoracic region the posterior layer is attached laterally to the angles of the ribs, and is covered by 

 the trapezius and by the rhomboidei. 



The anterior layer (Fig. 237) runs from the inner lip of the crest of the ilium to the twelfth 

 rib, and is attached internally to the transverse processes of the lumbar vertebrae. Its upper 

 margin forms a firmer tendinous band which passes from the transverse process of the first 

 lumbar vertebra to the twelfth rib and is known as the lumbocostal (external arcuate) ligament 

 (Fig. 251). The anterior layer is situated between the sacrospinalis and the quadratus lumborum, 

 and at the outer margin of the sacrospinalis the two layers unite and give origin to several of the 

 abdominal muscles. 



The thin nuchal fascia is situated beneath the trapezius and also partly beneath the rhom- 

 boidei. It is continuous below with the upper portion of the lumbodorsal fascia and externally 

 with the fascia of the neck, and the fasciae of the two sides are connected in the median line with 

 the nuchal ligament. 



[The trunk musculature is derived from the trunk myotomes of the embryo and is clearly divisible into two portions: 

 (1) the dorsal trunk musculature, derived from the dorsal portions of the myotomes and supplied by the dorsal (posterior) 

 branches of the spinal nerves; and (2) the ventral trunk musculature, developed from the ventral portions of the trunk 

 myotomes and supplied by the ventral branches of the spinal nerves. 



When considered from this standpoint, the muscles of the back as arranged above clearly form a somewhat hetero- 

 geneous group. The flat muscles are for the most part supplied by ventral branches of the spinal nerves or, in the 

 case of the trapezius, by a cranial nerve, a fact which at once distinguishes them from the long and short muscles 

 together with the splenii, which form the true dorsal musculature. The majority of the flat muscles are in reality muscles 

 of the upper limb and the trapezius is primarily part of the cranial musculature; they will be considered later in con- 

 nection with the other muscles of their groups. 



So far as the true dorsal musculature is concerned, comparative anatomy has shown that it is composed of two parallel 

 groups of muscles, a lateral one, which consists of muscles primarily passing from the transverse processes to the ribs, 

 and hence is termed the transversocostal group, and a more median one, whose muscles pass from the transverse to the 



